Cancer is a leading cause of death in the United States. In 2001, of the over half a million deaths in the United States, one in every four deaths was likely attributable to cancer. If all cancers were diagnosed at a localized stage, the five year survival rate would be over 95%. Overall, metastasis is considered by many to be the deadliest aspect of cancer.
It has been clinically noted that particular primary tumors tend to metastasize to specific distant organs. For example, prostate cancer often metastasizes to the bone, breast cancer may metastasize to the liver, melanomas tend to spread to lymph nodes, ovarian cancer metastasizes to other areas of the body including the lungs, brain, lymph and bones. Once a cancer has spread, it becomes much more lethal. No longer is a simple surgical intervention (e.g., to remove the primary tumor) an effective form of treatment. In addition, “blunt instruments” (e.g., radiation treatment and chemotherapy) affect not only cancerous cells, but also normal tissues throughout the body.
Cancer metastasis involves a series of sequential steps. After the initial transforming event, growth of neoplastic cells must be progressive. Extensive vascularization or angiogeneis also must occur, which allows blood vessels to grow into the tumor mass, bringing nourishment and allowing increased tumor growth. As the vascularization increases and the tumor grows, the thin walled venules or anastomoses of the capillary network allow for the penetration of cancer cells. These cells then may detach from the tumor mass and enter the circulation in a process called “embolization.” The majority of cancer cell aggregates that enter the circulation is destroyed, yet some of the aggregates migrate to distant capillary beds and begin a process called extravasation. During extravasation, tumor cells exit the capillary network, colonize a distant organ and create a secondary or metastatic tumor. This “homing,” or metastasis, of particular types of tumor cells to specific “target” organs provides further evidence that organ-specific markers exist.
The extravasation process is thought to begin with a type of adherence to the vascular walls—either by (i) attachment to specific proteins on the endothelial surface of the vasculature or (ii) a non-specific type of adhesion to the homing molecule at the target organ. In either case, there exists the possibility for multiple molecules and mechanisms of adherence of both homing molecules from primary tumors of different origins and for target molecules at the site of secondary metastatic tumors in specific organs.
Dreyer and Hood formulated the “Area Code Hypothesis” in the study of embryology and tissue differentiation. J. Supramol Struct. 1977;7(3-4);531-559. This hypothesis is concerned with the structure, function and regulation of cell-surface molecules that mediate recognition during embryogenesis. Ruoslahti and Pasqualini, who applied the area code hypothesis, developed a method that involved putting random peptide sequences in a phage display library, which then were injected into mice (see e.g. U.S. Pat. Nos. 5,622,699 and 6,232,287). Such “in vivo phage display” led to the identification of several molecular motifs, which localized to specific organs. The goal which Ruoslahti and Pasqualini hoped to achieve was a method to specifically attack metastatic tumors using the identified motifs. However, the physiologic basis for this targeting remains unknown, and neither the native homing molecules on the metastatic cell or the target molecule at the site of the secondary tumor have been identified, with singular exceptions.
In addition, Pasqualini and Ruoslahti expressed skepticism that organs, which filter a high blood volume would be amenable to the procedure they described, due to their ability to non-specifically capture a large number of blood borne peptides. Nevertheless, the clinical observation that particular primary tumors do home in on target organs in spite of their small volume of blood flow, prior to colonizing organs with high volumes, such as the liver, kidney or lungs is well documented. If these mechanisms could be identified, powerful new ways to study and treat cancer would be available. There is, therefore, a well recognized need to identify molecules that allow the homing of cancer cells in vivo. In the same vein, there is a need for a mechanism to identify molecules at distant sites that are targeted by metastasizing cells.